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We report on an unexpected domain of high Coulombic efficiency for electrochemical oxidation of aluminum in aqueous polysulfide solutions at high current density for the process:  Al + 3OH- → Al(OH)3 + 3e-. This high-efficiency domain, of importance to battery processes, includes aluminum oxidation in a wide range of solutions containing concentrated dissolved zerovalent sulfur. As expected at lower concentrations of dissolved sulfur, aluminum electrochemical oxidation is inefficient, due to various exothermic parasitic reactions, including:  Al(c) + yS x 2-(aq) + yH2O(l) ⇄ 1/2Al2S3(c) + yOH-(aq) + yHS-(aq), y = 1.5/(x − 1), and Al(c) + 1.5S2 2-(aq) + 3H2O(1) ⇄ Al(OH)3(amorph) + 3HS-(aq). However, at high polysulfide and sulfur concentrations, the Coulombic efficiency can approach 100%. This domain of high efficiency is correlated to an observed cathodic shift with increasing sulfur concentration, leading to improved chemical passivation at the aluminum surface.

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Experimental SectionSyntheses. (dmq)Cu2Br2 (1). Anhydrous copper(II) bromide (1.03 g, 4.62 mmol) was added to 30 mL of 95% ethanol. The mixture was warmed, and the copper salt dissolved upon the addition of 5 mL of distilled water to give a green/black solution. The 2,3-dimethylquinozaline (0.722 g, 4.56 mmol) was dissolved in 30 mL of warm 95% ethanol. The warm copper solution was then added to the warm, colorless dmq solution to give a reddish brown solution. Upon evaporation of the solvent, reddish brown crystals began to form. The crystals were collected by filtration and allowed to air dry. Yield: 0.577 g, 56%.